1. Field of the Invention
The present invention generally relates to image capture devices and methods and, more particularly, to capturing an image of an optical code symbol to be electro-optically read using an imaging array, and especially at a low ambient light illumination level.
2. Description of the Related Art
Optical codes are patterns made up of image areas having different light-reflective or light-emissive properties, which are typically assembled in accordance with a priori rules. The term “barcode” is sometimes used to describe certain kinds of optical codes. The optical properties and patterns of optical codes are selected to distinguish them in appearance from the background environments in which they are used. Systems for identifying or extracting data from optical codes are sometimes referred to as “optical code readers” of which barcode scanners are one type. Optical code readers are used in both fixed or portable installations in many diverse environments such as in stores for check-out services, in manufacturing locations for work flow and inventory control, and in transport vehicles for tracking package handling. The optical code can be used as a rapid, generalized means of data entry, for example, by reading a target barcode from a printed listing of many barcodes. In some uses, the optical code reader is connected to a portable data processing device or a data collection and transmission device. Frequently, the optical code reader includes a hand-held sensor which is manually directed at a target code.
Most conventional optical scanning systems are designed to read one-dimensional barcode symbols. The barcode is a pattern of variable-width rectangular bars separated by fixed or variable width spaces. The bars and spaces have different light-reflecting characteristics. One example of a one-dimensional barcode is the UPC/EAN code used to identify, for example, product inventory. An example of a two-dimensional or stacked barcode is the PDF417 barcode. A description of PDF417 barcode and techniques for decoding it are disclosed in U.S. Pat. No. 5,635,697. Another conventional optical code is known as “MaxiCode”. It consists of a central finder pattern or bull's eye center and a grid of hexagons surrounding the central finder. It should be noted that the aspects of the inventions disclosed in this patent application are applicable to optical code readers, in general, without regard to the particular type of optical codes which they are adapted to read. The invention described is also applicable to some associated image recognition or analysis.
Most conventional scanning systems generate one or more beams of laser light which reflects off a barcode symbol and back to the system. The system obtains a continuous analog waveform corresponding to the light reflected by the code along one or more scan lines of the system. The system then decodes the waveform to extract information from the barcode. A system of this general type is disclosed, for example, in U.S. Pat. No. 4,251,798. A beam scanning system for detecting and decoding one-and two-dimensional barcodes is disclosed in U.S. Pat. No. 5,561,283.
Many scanning systems in use today employ a scanning laser beam. Some such systems are deployed in hand-held devices which may be manually pointed at a target. Often an individual system is a component of a much larger network including other scanners, computers, cabling, data terminals, etc.
Barcodes can also be read by employing imaging modules in the systems. For example, an imaging module may be employed in a device which contains a two-dimensional array of cells or photosensors which correspond to image elements or pixels in a field of view of the device. Such an imaging module may include a two-dimensional or area charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) array, associated circuits for producing electronic signals corresponding to the two-dimensional array of pixel information over the field of view, and a single light emitting diode (LED) for providing background light illumination.
It is therefore known to use a CCD for capturing a monochrome image of a barcode symbol to be read as, for example, disclosed in U.S. Pat. No. 5,703,349. It is also known to use a CCD with multiple buried channels for capturing a full color image of a target as, for example, disclosed in U.S. Pat. No. 4,613,895.
Although generally satisfactory for its intended purpose, the known imaging module tends to have poorer performance as the size of the module decreases. In many applications, especially in mobile, portable data collection terminals, it is desired to have the terminal as light in weight, as energy efficient, and as compact as possible. However, such weight, size, and energy constraints configure the module with a single light emitting diode (LED) for providing background light illumination. This limited illumination, especially in dimly lit ambient environments, tends to degrade image capture performance.